This invention relates to a semiconductor photocoupler.
A semiconductor photocoupler is used for transmitting information between circuits electrically isolated from each other, and is usually called an optically coupled isolator. Optically coupled isolators on the market are of current-light-current type in information transmission, where an instantaneous magnitude of a current flowing in a phototransistor(or a photodiode) of the secondary circuit is proportional to that driving an LED(light emitting diode) of the primary circuit.
The inventor of the present invention has disclosed another type of optically coupled isolator in his prior invention. The prior invention has been patented as a Japanese Patent No. 1,455,290 entitled "Semiconductor Photocoupler" and published as publication No. 62-60828/'87 on Dec. 18, 1987, listing Tetsuro Kato as inventor and NEC as the assignee.
In the prior invention, two light emitting LEDs and a light receiving LED are disclosed, whereby each of the light emitting LEDs emit light of mutually different wavelengths when driven by separate current sources. The light receiving LED, upon receiving this light, experiences a change in capacitance, the magnitude of which depends on which of the light emitting LEDs is ON and the wavelength of the light emitted therefrom information transmission photocoupler is a current-light-capacitance change, and the capacitance change ceases when the light received at a light receiving element of the secondary circuit disappears. In theory, the prior invention utilizes a deep impurity level in and around the neighborhood of the p-n junction of a light receiving element.
Some impurities located deep in a semiconductor have a very large optical capture cross-section(.sigma..sub.n for electron and .sigma..sub.p for hole) for certain wavelengths. Some electrons (or holes) thus captured are seldom released by thermal excitation at room temperature. Therefore, such a trap center (capture cross-section) has an occupation rate of electrons(or holes) of the level dependent only on the wavelengths of the received light. When such a trap center is formed in a p-n junction, the junction capacity is dependent upon the wavelength of the received light, and the junction maintains the capacitance after the received light disappears.
H. Kukimoto et al have disclosed in Physical Review B, vol. 7 No. 6, pp. 2486.about.2507 Mar. 15, 1973 that oxygen doped in a p-n junction or in a p type Zn, O doped GaP red light LED makes such a trap center.
As for oxygen doped in GaP, the capture cross-section .sigma..sub.p for capturing valence band electron at the oxygen level begins at 1.4 eV of received light energy and increases as the received light energy increases to about 1.7.times.10.sup.-16 cm.sup.2 at 1.8 eV(.lambda..apprxeq.689 nm), while the capture cross-section .sigma..sub.n for releasing electron trapped at the oxygen level to the conduction band begins at 0.8 eV, increases as received light energy increases, reaches to a maximum of 3.4.times.10.sup.-16 cm.sup.2 at 1.2 eV, and then decreases as received light energy increases. Therefore, this oxygen level will become neutral by received light of energy higher than 1.7 eV because it captures electrons at the oxygen level, and will become positively charged by receiving light at an energy level of about 1.2 eV.
Since the oxygen level is sufficiently deep (0.9 eV from the conduction band), the trapped electrons are thermally released only in a small quantity. Therefore, the GaP red LED doped by Zn, O makes a light receiving element of a semiconductor photocoupler working on current-light-capacitance in information transmission and having a memory action.
In the prior invention, a semiconductor photocoupler is composed of a light receiving element of a GaP red LED doped by Zn, O, and two light emitting elements, one light emitting element being a GaP red LED (1.77 eV), and the other light emitting element being a Si doped GaAs LED (1.29 eV).
The semiconductor photocoupler of the prior invention transmits a binary signal indicating which light emitting element of the two light emitting elements is energized as a binary capacitance change in the light receiving element and maintains the capacitance corresponding to the lastly received wavelength. This photocoupler can be used as a coupler of binary signals, but can not be used as a coupler of analog signals.
As there is one-to-one correspondence between the current in the light emitting element and that in the light receiving element in a conventional optically coupled isolator, the isolator can be used as a coupler of analog signals. If the current-light-capacitance type photocoupler is to be used as a coupler of analog signals, there must be one-to-one correspondence between the current in the light emitting element and the capacitance of the light receiving element.